1. Field of the Invention
The present invention relates to an image processing device, an imaging device, a computer, an image processing method and a computer readable non-transitory medium, and in particular, relates to a restoration filter used in point image restoration processing.
2. Description of the Related Art
In a subject image photographed through an imaging optical system, a so-called point spread phenomenon that a point subject has a minute spread is sometimes seen because of influence of diffraction or aberration due to the imaging optical system. A function indicating a response to a point light source of an optical system is called a point spread function (PSF), and is known as a parameter that controls resolution degradation (blur) of a photographed image.
A photographed image whose image quality is degraded due to the point spread phenomenon can recover from image quality degradation by receiving point image restoration processing based on the PSF. The point image restoration processing is processing of obtaining a degradation characteristic (point image characteristic) due to the aberration of a lens (optical system) or the like beforehand and canceling a point spread of the photographed image by image processing using a restoration filter (recovery filter) according to the point image characteristic.
For the point image restoration processing, various contrivances have been proposed. For example, Japanese patent application publication No. 2003-172873 discloses a correction data preparation method capable of compensating degradation of an optical performance off an optical axis by using a rotation-symmetrical sharpness recovery filter instead of a rotation-asymmetrical sharpness recovery filter in light of symmetry of the spread of the PSF. Also, Japanese patent application publication No. 2012-129932 discloses an image processing method of correcting color shift due to chromatic aberration of magnification by generating a recovery image using an image recovery filter based on an optical transfer function and reducing the color shift of the recovery image.
Since this point spread phenomenon variously changes depending on photographing conditions such as a diaphragm, a zoom position and a subject distance and an image height position and the like, even for a single imaging system, a data amount of the PSF and the like required for the point image restoration processing becomes huge. Then, Japanese patent application publication No. 2012-073691 discloses an image processing device for reducing a storage amount of data required for reconfiguration of an optical transfer function of an imaging optical system, to reduce a storage data amount of the PSF.
As described above, while a huge amount of storage data is required in order to appropriately perform the point image restoration processing utilizing a restoration filter, reduction of a storage data amount is one of big problems. On the other hand, storage of the restoration filter in prescribed storage means for the point image restoration processing is extremely strongly demanded, and by storing the prepared restoration filter and reading the stored restoration filter upon utilization, calculation loads can be reduced and a processing speed can be accelerated. However, since the restoration filter is configured by many taps and a filter coefficient is allocated to each tap, the storage loads are extremely large.
In the apparatuses disclosed in Japanese patent application publication No. 2003-172873 and Japanese patent application publication No. 2012-129932, no device to effectively reduce the data amount of the restoration filter of the point image restoration processing while maintaining point image restoration accuracy is employed in particular. That is, in Japanese patent application publication No. 2003-172873, a device for improving image quality degradation in an area away from an optical axis by a sharpening filter having a rotation-symmetrical coefficient is proposed; however, since the sharpening filter having the rotation-symmetrical coefficient is approximately applied in the case where a blur characteristic of the optical system is rotation-asymmetrical, accuracy of point image restoration drops. Also, in Japanese patent application publication No. 2012-129932, no device is suggested regarding reduction of the storage data amount of the point image restoration filter.
Also, while Japanese patent application publication No. 2012-073691 discloses a technology of reducing a storage amount of data required for reconfiguration of the optical transfer function of the imaging optical system, no device to directly reduce the storage data amount of the restoration filter for the point image restoration processing is disclosed or suggested. That is, the technology disclosed in Japanese patent application publication No. 2012-073691 is for reducing an information amount of the optical transfer function for preparing an image recovery filter, but is not for reducing the storage amount of the prepared image recovery filter itself.